Computer-generated lithograms are two-dimensional matrices, preferably having very small dimensions, which consist of individual dots with different optical properties. The lithograms are preferably formed as holograms, but microtexts or micropictures that can be read directly, that is to say without diffraction of the incident light, can also be generated with the lithograms.
By means of illuminating a computer-generated hologram, for example with a coherent electromagnetic wave, in particular a light wave, images and/or data are reproduced by diffraction in transmission or reflection. The different optical properties of the individual dots can be reflective properties, for example arising from surface topography, varying optical path lengths in the material of the storage medium (refractive indices) or color values of the material.
The optical properties of the individual dots are calculated by the computer, and these are thus what are known as computer-generated holograms (CGH) or, generally, computer-generated lithograms. With the aid of the focused write beam, the individual dots of the lithograms are written into the material during writing, the focus lying in the region of the surface or in the material of the storage medium. In the region of the focus, focusing has the effect of a small area of action on the material of the storage medium, so that a large number of dots of the lithogram can be written in a small region.
By means of scanning an intensity-modulated write beam, an area with an irregular dot distribution, the computer-generated lithogram, is thus produced. This can be employed for marking, identifying and individualizing any desired objects.
During the production of lithograms, a resolution of about 25 000 dpi is preferred. In addition, in computer-generated lithography, only comparatively small areas are inscribed. These are, for example, 1 to 5 mm2 in size, other sizes also being possible. The accuracy of the writing pattern in the case of a lithograph for producing digital holograms of, for example, 1000×1000 dots on an area of 1×1 mm2 must be about ±0.1 μm in both orthogonal directions. Furthermore, the writing speed should be approximately 1 Mpixel/s, in order that in each case a lithogram can be written in the time of about 1 s or less.
The computer-generated lithograms explained previously can carry different information. Firstly, the computer-generated lithogram can carry an item of holographic information which, when read, that is to say during reproduction, reproduces a directly legible item of information (text, image, graphic). Likewise, the information read out can be encrypted, in particular digitized. The reproduced hologram then contains, for example, a digital matrix similar to a one-dimensional or two-dimensional bar code. Finally, the dot distribution of the computer-generated lithogram can intrinsically have an item of directly legible information with very small dimensions, for example a microtext or a micropicture. For the purpose of detection, magnification of the directly legible microtext is necessary, that is to say a procedure different from reading the digital hologram. The microtext and/or the micropicture itself can in turn be encrypted. For example, a micropicture can itself represent an item of digital information like a one-dimensional or two-dimensional bar code. Of course, a lithogram can contain both holographic information and directly legible information, that is to say a combination of all the possibilities illustrated.
In order to read computer-generated holograms, simple handheld readers are known from the prior art. As a rule, these make use of a light beam to generate the reproduced hologram on a screen.
Furthermore, nonportable, permanently installed readers are known, which display the information read on a computer monitor.
The technique of using computer-generated lithograms has recently led to various applications in which individual products are provided with small labels, on which a hologram is written in each case. The lithograms used can be individualized and/or coded in a number of stages; any desired combination of microtexts and micropictures, directly legible holographic information and information stored in coded form in the hologram, for example digitized information, can be combined with one another. A [lacuna] for checking the various items of information with a reader has previously not been possible, in particular not with a handheld reader.